Microbes are everywhere in the world and some of them are positive, but others may be fatal. According to the list of major concerns about health from the World Health Organization (WHO), many health concerns are caused by micro-organisms, which contaminate water and sanitation services and infect thousands of people. Therefore, some treatments such as cleaning, disinfection, and sterilization of wastewater and air are necessary. Some natural products or small molecules are used to prevent the spread and growth of microbes, but various unpleasant side effects (such as releasing biocides into the environment) are still significant concerns.
Compared to low molecular weight antibacterial agents, antimicrobial polymers are chemically stable macromolecules with reduced residual toxicity, prolonged lifetime, and improved antimicrobial efficiency. Antimicrobial polymers can be classified into three types: (a) polymeric biocides, (b) biocidal polymers, and (c) biocide-releasing polymers. Siedenbiedel, F.; Tiller, J. C. Polymers 2012, 4, 46. In essence, the active ingredients of the polymeric biocides and biocide-releasing polymers are small molecules possessing antimicrobial activity. On the other hand, biocidal polymers can be an effective tool to kill microbes and may be less likely to result in evolved drug resistance. Typical representatives of biocidal polymers are cationic polymers containing quaternary ammonium groups. In terms of evolutionary history, it is difficult for microbes to produce drug resistance to such species because polycations interact with the microbial cells carrying a negative charge.
To obtain permanent, non-leaching antimicrobial polymeric materials, specific antibacterial small molecules (e.g., 1,2-benzisothiazolin-3-one (BIT)) and small or macro cationic molecules (e.g., trimethylguanidine or tetramethylguanidine (TMG) or polyhexamethylene guanidine (PHMG)) can be immobilized to polymers.
For specific antibacterial small molecules, U.S. Pat. No. 9,371,479 discloses a technique for encapsulating a biocide such as BIT into carrier materials. After a predetermined time, biocides are released from the carrier materials and then kill the microbes. U.S. Pat. No. 9,392,786 discloses BIT serving as an ingredient to control pest. But this physically blended BIT material is not environment-friendly and increases the risk of drug resistance.
For small or macro cationic molecules, U.S. Patent Application Publication 2004/0115160 discloses that quaternary ammonium esters are dissolved in water or other solvent for disinfection and preservation of devices. Cationic molecules present antimicrobial properties as they universally interact with negatively charged microbial membranes via electrostatic interactions. A variety of methods were used to fabricate cationic polymers in the antimicrobial field because cationic polymers do not have to interact with specific targets in microbes and can effectively suppress bacterial drug resistance. For example, U.S. Patent Application Publication 2011/0150977 discloses biodegradable cationic block copolymers containing quaternary amine group prepared by ring-opening polymerization (ROP), which can form aqueous mixtures suitable for antimicrobial applications. Through the ROP method, U.S. Patent Application Publication 2012/0251607 and U.S. Patent Application Publication 2012/0251608 also disclose certain antimicrobial polycarbonates containing quaternary amine groups. U.S. Patent Application 2012/0195849 discloses certain novel polymers (SMAMPs) containing a guanidine group or quaternary amine group prepared by ring-opening metathesis polymerization (ROMP), which kill microbes through mimicking the structure of natural antimicrobial peptides (AMPs). U.S. Patent Application Publication 2010/0240799 discloses an application of PHMG in the antimicrobial film. The PHMG serves as film-forming composition via physical blending and also exhibits antimicrobial property. U.S. Pat. Nos. 7,282,538 and 7,531,225 disclose a functional polyolefin master batch containing guanidine salt oligomer by radical initiator. For these methods, physical blending results in quickly reduced bactericidal efficiency over service time due to the diffusion of the physically mixed antimicrobial agents. Polymerization of monomers containing guanidine group or quaternary amine may limit the use of available types and number of cationic monomer, which may also result in the loss of flexibility or range of polymer properties, such as mechanical properties.
Due to the above-mentioned limitations, there still exists an urgent need for an efficient and convenient approach to prepare coating materials with long-lasting antimicrobial properties and to enable fine-tuning of material properties such as mechanical properties.